SYNTAX: condition on the application of transformations, which states that, if a transformation can in principle be applied to two constituents in the structure, it has to be applied to the one that is superior. The formal definition (from
Chomsky (1973)) is as follows:

No rule can involve X,Y in the structure
...X...[...Z...-WYV...]...
where the rule applies ambiguously to Z and Y
and Z is superior to Y

EXAMPLE: given that wh-movement can apply only once
in deriving the s-structure of a sentence,
the superiority condition predicts that in structure (i) what cannot be moved: who is superior to what. The contrast in (ii) shows this
prediction to be correct.